EX-801 Electrical Drives - Jun 2025 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDraw the block diagram of electrical drives and explain the function of each block.
   How to answer

   • Define electrical drive and list main blocks: source, power modulator, motor, load, control and sensing unit
   • Draw labelled block diagram showing power and signal flow
   • Explain function of each block briefly
   • Mention example of closed-loop operation
2. 7 Marks
   bCompare AC and DC drives and write the advantages of electrical drives.
   How to answer

   • Define AC drives and DC drives with examples
   • Compare on basis of power source, converter, maintenance, cost, control
   • List advantages of electrical drives: efficiency, controllability, reliability
   • Conclude with application suitability
3. 7 Marks
   aExplain with circuit diagram principle and working of chopper control of separately DC motor.
   How to answer

   • Show chopper circuit for separately excited DC motor with source, chopper, freewheeling diode
   • Explain principle of variable average voltage by duty ratio control
   • Describe motoring and operation in quadrants with waveforms
   • Mention speed control relation and advantages
4. 7 Marks
   bExplain in brief the different types of braking employed in DC motors.
   How to answer

   • Define braking in DC drives
   • Explain regenerative, dynamic/rheostatic and plugging braking
   • Give circuit condition and energy flow for each
   • Compare applications and limitations
5. 7 Marks
   aWith a neat diagram, explain the V/f control of the three-phase induction motor. What is the range of speed control?
   How to answer

   • Draw power circuit with rectifier-inverter feeding induction motor
   • Explain constant V/f principle to maintain flux
   • Describe below and above base speed operation with characteristics
   • State speed control range and limitations
6. 7 Marks
   bA 2.8 kW, 400V, 50Hz, 4 pole, 1370 rpm, delta connected squirrel cage induction motor has the following parameters referred to the stator: $R_s = 2$ ohm, $R'_r = 5$ ohm, $X_s = X'_r = 5$ ohm, $X_m = 80$ ohm. Motor speed is controlled by stator voltage control. When driving a fan load it runs at rated speed at rated voltage. Calculate motor terminal voltage, current and torque at 1200 rpm.
   How to answer

   • Calculate synchronous speed, rated slip and rated torque from 2.8 kW, 1370 rpm
   • Use fan load law $T \propto N^2$ to find torque at 1200 rpm
   • Apply stator voltage control equivalent circuit with given $R_s, R'_r, X_s, X'_r, X_m$ to find reduced voltage
   • Compute stator current at 1200 rpm from equivalent circuit
7. 7 Marks
   aDescribe in detail about speed control of self-controlled synchronous motor drives.
   How to answer

   • Define self-controlled vs true synchronous operation
   • Describe inverter-fed drive with rotor position sensing
   • Explain speed control by frequency and voltage control
   • Mention characteristics and applications
8. 7 Marks
   bExplain the working principle of PMSM. (Permanent Magnet Synchronous Motor) motors.
   How to answer

   • Define PMSM construction with permanent magnet rotor
   • Explain rotating field interaction and synchronous torque production
   • Describe back EMF and control requirement
   • List features and applications
9. 7 Marks
   aExplain the switched reluctance motor with a neat diagram and describe its principle of working.
   How to answer

   • Draw cross-section of switched reluctance motor with stator and rotor poles
   • Explain reluctance torque due to minimum reluctance alignment
   • Describe sequential phase excitation and speed control
   • Mention advantages and drawbacks
10. 7 Marks
    bExplain with the help of the construction, operation and switching scheme of Brushless D.C. motor.
    How to answer

    • Describe construction: trapezoidal PM rotor, stator, Hall sensors
    • Explain electronic commutation and switching sequence
    • Describe operation with voltage and current waveforms
    • State speed control and applications
11. 7 Marks
    aDescribe the static Kramer drive for speed control of the induction motor.
    How to answer

    • Draw schematic of static Kramer drive with slip-ring motor, rectifier, inverter
    • Explain slip power recovery principle for sub-synchronous speed
    • Derive speed control range and power flow
    • Mention limitations and applications
12. 7 Marks
    bDescribe the static Scherbius drive for speed control of the induction motor.
    How to answer

    • Draw schematic of static Scherbius drive with cycloconverter/inverter
    • Explain bidirectional slip power flow for sub and super-synchronous speed
    • Compare with Kramer drive for speed range
    • Mention applications
13. 7 Marks
    aDescribe various drive schemes used in cement industries.
    How to answer

    • List loads in cement plant: kiln, crushers, fans, conveyors
    • Describe drive requirements: starting torque, speed range
    • Explain AC/DC drive schemes adopted
    • Mention energy saving and control aspects
14. 7 Marks
    bDescribe various drive schemes used in steel industries.
    How to answer

    • List loads in steel plant: rolling mills, cranes, fans, pumps
    • Describe duty, reversing and high torque requirements
    • Explain drive schemes for rolling mills and auxiliaries
    • Mention modern AC drives usage
15. 14 Marks
    Write short notes (any two):

    1. i)Regenerative braking in induction motors
    2. ii)Why starters are used in DC motors?
    3. iii)Constant torque and constant power drive
    How to answer

    • For chosen two topics give definition and principle
    • Explain with circuit/diagram and key equations
    • Mention operating conditions and control
    • List applications and limitations